FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsMedium
A pilot operates a normally aspirated engine at high power with an excessively lean mixture on a hot day using the minimum octane fuel specified. Under these conditions, the engine is most susceptible to which abnormal combustion event?
- ADetonation caused by excessive cylinder head temperatures and pressures
- BVapor lock in the fuel lines
- CCarburetor icing due to the venturi effect
- DPre-ignition caused by a fouled spark plug
Show answer & explanationAnswer & explanation
Correct answer: A. Detonation caused by excessive cylinder head temperatures and pressures
High power settings combined with an excessively lean mixture and high temperatures raise cylinder head temperature and internal cylinder pressure, which can cause the fuel/air charge to spontaneously explode rather than burn smoothly — this is detonation. Detonation is worsened by lean mixtures, high power, and inadequate octane fuel.
Why the other options are wrong
- B. Vapor lock involves fuel boiling in the lines, unrelated to mixture leaning or combustion timing.
- C. Carburetor icing is associated with reduced power settings and moist air, not high-power lean operation.
- D. Pre-ignition is caused by a hot spot such as a glowing carbon deposit igniting the mixture early, not primarily by lean mixture at high power.
Detonation
Uncontrolled, explosive combustion of the fuel/air mixture in the cylinder caused by excessive heat and pressure, often from lean mixtures at high power.
- Causes: high power, lean mixture, high CHT, low-octane fuel
- Symptoms: rough running, loss of power, engine damage
- Remedy: enrich mixture, reduce power, increase airspeed to cool engine
- Different from pre-ignition, which is caused by a hot spot igniting mixture early
Memory trick: Lean + Hot + High Power = Detonation Disaster.